Hematopoietic stem cell transduction and amplification in large animal models

In vivo selection of anti-HIV-1 gene-modified human hematopoietic stem/progenitor cells to enhance engraftment and HIV-1 inhibition

Hematopoietic stem/progenitor cell (HSPC)-based anti-HIV-1 gene therapy holds great promise to eradicate HIV-1 or to provide long-term remission through a continuous supply of anti-HIV-1 gene-modified cells without ongoing antiretroviral therapy. However, achieving sufficient engraftment levels of anti-HIV gene-modified HSPC to provide therapeutic efficacy has been a major limitation. Here, we report an in vivo selection strategy for anti-HIV-1 gene-modified HSPC by introducing 6-thioguanine (6TG) chemoresistance through knocking down hypoxanthine-guanine phosphoribosyl transferase (HPRT) expression using RNA interference (RNAi). We developed a lentiviral vector capable of co-expressing short hairpin RNA (shRNA) against HPRT alongside two anti-HIV-1 genes: shRNA targeting HIV-1 co-receptor CCR5 and a membrane-anchored HIV-1 fusion inhibitor, C46, for efficient in vivo selection of anti-HIV-1 gene-modified human HSPC. 6TG-mediated preconditioning and in vivo selection significantly enhanced engraftment of HPRT-knockdown anti-HIV-1 gene-modified cells (>2-fold, p < 0.0001) in humanized bone marrow/liver/thymus (huBLT) mice. Viral load was significantly reduced (>1 log fold, p < 0.001) in 6TG-treated HIV-1-infected huBLT mice compared to 6TG-untreated mice. We demonstrated that 6TG-mediated preconditioning and in vivo selection considerably improved engraftment of HPRT-knockdown anti-HIV-1 gene-modified HSPC and repopulation of anti-HIV-1 gene-modified hematopoietic cells in huBLT mice, allowing for efficient HIV-1 inhibition.

Lentiviral Transduction of Nonhuman Primate Hematopoietic Stem and Progenitor Cells

The nonhuman primate (NHP) animal model is an important predictive preclinical model for developing gene and cell therapies. It is also an experimental animal model used to study hematopoietic stem and progenitor cell (HSPC) biology, with the capability of serving as a step for the translation of the basic research concepts from small animals to humans. Lentiviral vectors are currently the standard gene delivery vehicles for transduction of HSPCs in the clinical setting. They have proven to be less genotoxic and more efficient than the previously used murine γ-retroviruses. Transplantation of lentiviral vector-transduced HSPCs into autologous macaques has been well developed over the past two decades. In this chapter, we provide detailed methodologies for lentiviral vector transduction of rhesus macaque HSPCs, including production and titration of lentiviral vector, purification of CD34⁺ HSPCs, and lentiviral vector transduction and assessment.

Combinatorial hematopoietic stem cell transplantation and vaccination reduces viral pathogenesis following SHIV89.6P-challenge

Development of curative approaches for HIV-1 infected patients requires novel approaches aimed at eliminating viral reservoirs and replacing potential target cells with infection-resistant immune cell populations. We have previously shown that autologous transplantation of genetically modified hematopoietic stem cells (HSCs) with lentiviral vectors encoding the mC46-fusion inhibitor results in a significant reduction in viral pathogenesis following challenge with the highly pathogenic dual tropic, SHIV89.6P strain. In this study, we used a combinatorial approach in which following engraftment of genetically modified HSCs, pigtailed macaques were vaccinated with a previously developed vaccinia-based vaccine expressing SIV-Gag, Pol. Using this dual therapy approach, lower viremia was detected in both the acute and chronic phase of disease with levels reaching near the lower limits of detection. In comparison with macaques receiving HSCT only, the combination approach resulted in a further log decrease in plasma viremia. Similar to our previous studies, positive selection of all CD4(+) T-cell subsets was observed; however, higher gene-modified CD4(+) T-cell levels were observed during the chronic phase when vaccination was included suggesting that combining vaccination with HSCT may lower the necessary threshold for achieving viremic control.

Genetic modification of mouse bone marrow by lentiviral vector-mediated delivery of hypoxanthine-Guanine phosphoribosyltransferase short hairpin RNA confers chemoprotection against 6-thioguanine cytotoxicity

We have recently developed a novel and highly efficient strategy that exclusively uses the purine analog 6-thioguanine (6TG) for both pretransplantation conditioning and post-transplantation chemoselection of hypoxanthine-guanine phosphoribosyltransferase (HPRT)-deficient bone marrow (BM). In a mouse BM transplantation model, combined 6TG preconditioning and in vivo chemoselection consistently achieved >95% engraftment of HPRT-deficient donor BM and long-term reconstitution of histologically and immunophenotypically normal hematopoiesis in both primary and secondary recipients, without significant toxicity and in the absence of any other cytotoxic conditioning regimen. To translate this strategy for combined 6TG conditioning and chemoselection into a clinically feasible approach, it is necessary to develop methods for genetic modification of normal hematopoietic stem cells (HSC) to render them HPRT-deficient and thus 6TG-resistant. Here we investigated a strategy to reduce HPRT expression and thereby confer protection against 6TG myelotoxicity to primary murine BM cells by RNA interference (RNAi). Accordingly, we constructed and validated a lentiviral gene transfer vector expressing short-hairpin RNA (shRNA) that targets the murine HPRT gene. Our results showed that lentiviral vector-mediated delivery of HPRT-targeted shRNA could achieve effective and long-term reduction of HPRT expression. Furthermore, in both an established murine cell line as well as in primary murine BM cells, lentiviral transduction with HPRT-targeted shRNA was associated with enhanced resistance to 6TG cytotoxicity in vitro. Hence this represents a translationally feasible method to genetically engineer HSC for implementation of 6TG-mediated preconditioning and in vivo chemoselection.

Positive selection of mC46-expressing CD4+ T cells and maintenance of virus specific immunity in a primate AIDS model

Despite continued progress in the development of novel antiretroviral therapies, it has become increasingly evident that drug-based treatments will not lead to a functional or sterilizing cure for HIV(+) patients. In 2009, an HIV(+) patient was effectively cured of HIV following allogeneic transplantation of hematopoietic stem cells (HSCs) from a CCR5(-/-) donor. The utility of this approach, however, is severely limited because of the difficulty in finding matched donors. Hence, we studied the potential of HIV-resistant stem cells in the autologous setting in a nonhuman primate AIDS model and incorporated a fusion inhibitor (mC46) as the means for developing infection-resistant cells. Pigtail macaques underwent identical transplants and Simian-Human Immunodeficiency Virus (SHIV) challenge procedures with the only variation between control and mC46 macaques being the inclusion of a fusion-inhibitor expression cassette. Following SHIV challenge, mC46 macaques, but not control macaques, showed a positive selection of gene-modified CD4(+) T cells in peripheral blood, gastrointestinal tract, and lymph nodes, accounting for >90% of the total CD4(+) T-cell population. mC46 macaques also maintained high frequencies of SHIV-specific, gene-modified CD4(+) T cells, an increase in nonmodified CD4(+) T cells, enhanced cytotoxic T lymphocyte function, and antibody responses. These data suggest that HSC protection may be a potential alternative to conventional antiretroviral therapy in patients with HIV/AIDS.

Large animal models for foamy virus vector gene therapy

Foamy virus (FV) vectors have shown great promise for hematopoietic stem cell (HSC) gene therapy. Their ability to efficiently deliver transgenes to multi-lineage long-term repopulating cells in large animal models suggests they will be effective for several human hematopoietic diseases. Here, we review FV vector studies in large animal models, including the use of FV vectors with the mutant O6-methylguanine-DNA methyltransferase, MGMTP140K to increase the number of genetically modified cells after transplantation. In these studies, FV vectors have mediated efficient gene transfer to polyclonal repopulating cells using short ex vivo transduction protocols designed to minimize the negative effects of ex vivo culture on stem cell engraftment. In this regard, FV vectors appear superior to gammaretroviral vectors, which require longer ex vivo culture to effect efficient transduction. FV vectors have also compared favorably with lentiviral vectors when directly compared in the dog model. FV vectors have corrected leukocyte adhesion deficiency and pyruvate kinase deficiency in the dog large animal model. FV vectors also appear safer than gammaretroviral vectors based on a reduced frequency of integrants near promoters and also near proto-oncogenes in canine repopulating cells. Together, these studies suggest that FV vectors should be highly effective for several human hematopoietic diseases, including those that will require relatively high percentages of gene-modified cells to achieve clinical benefit.

Stem cell selection in vivo using foamy vectors cures canine pyruvate kinase deficiency

Background

Hematopoietic stem cell (HSC) gene therapy has cured immunodeficiencies including X-linked severe combined immunodeficiency (SCID-X1) and adenine deaminase deficiency (ADA). For these immunodeficiencies corrected cells have a selective advantage in vivo, and low numbers of gene-modified cells are sufficient to provide therapeutic benefit. Strategies to efficiently transduce and/or expand long-term repopulating cells in vivo are needed for treatment of diseases that require higher levels of corrected cells, such as hemoglobinopathies. Here we expanded corrected stem cells in vivo in a canine model of a severe erythroid disease, pyruvate kinase deficiency.

Methodology/Principal Findings

We used a foamy virus (FV) vector expressing the P140K mutant of methylguanine methyltransferase (MGMTP140K) for in vivo expansion of corrected hematopoietic repopulating cells. FV vectors are attractive gene transfer vectors for hematopoietic stem cell gene therapy since they efficiently transduce repopulating cells and may be safer than more commonly used gammaretroviral vectors. Following transplantation with HSCs transduced ex vivo using a tri-cistronic FV vector that expressed EGFP, R-type pyruvate kinase, and MGMTP140K, we were able to increase marking from approximately 3.5% to 33% in myeloid long-term repopulating cells resulting in a functional cure.

Conclusions/Significance

Here we describe in one affected dog a functional cure for a severe erythroid disease using stem cell selection in vivo. In addition to providing a potential cure for patients with pyruvate kinase deficiency, in vivo selection using foamy vectors with MGMTP140K has broad potential for several hematopoietic diseases including hemoglobinopathies.

Imaging stem cell-derived persistent foci after in vivo selection of lentiviral MGMT-P140K transduced murine bone marrow cells

The engraftment of hematopoietic stem cells (HSCs) after drug resistance gene transfer and drug selection may recapitulate stress response hematopoiesis, but the processes remain elusive. Homing, trafficking, and localization of transduced cells and the impact of insertion site on focal expansion have not been well characterized. With the goal of optimizing and understanding these processes under conditions of low multiplicity of infection (MOI) lentiviral gene transfer, we used drug resistance gene O(6)-methylguanine-DNA methyltransferase (MGMT)-P140K and in vivo selection to enrich for transduced and transgene-expressing HSCs. To systemically monitor homing, trafficking, and expansion after transplantation and drug selection over time, we linked MGMT-P140K to the firefly luciferase gene in lentiviral self-inactivating vectors. Periodic bioluminescence imaging (BLI) of transplanted recipients was followed for up to 9 months after both primary and secondary transplantation. Initial dispersion and widespread early homing and engraftment were transient, followed by detection of persistent and discrete foci at stable tissue sites after in vivo drug selection. From these studies, we concluded that drug resistance gene transfer followed by early or late drug selection can result in stable gene expression and cell expansion in persistent foci of transduced bone marrow cells that often remain in fixed sites for extended periods of time.

Computational models of HIV-1 resistance to gene therapy elucidate therapy design principles

Gene therapy is an emerging alternative to conventional anti-HIV-1 drugs, and can potentially control the virus while alleviating major limitations of current approaches. Yet, HIV-1's ability to rapidly acquire mutations and escape therapy presents a critical challenge to any novel treatment paradigm. Viral escape is thus a key consideration in the design of any gene-based technique. We develop a computational model of HIV's evolutionary dynamics in vivo in the presence of a genetic therapy to explore the impact of therapy parameters and strategies on the development of resistance. Our model is generic and captures the properties of a broad class of gene-based agents that inhibit early stages of the viral life cycle. We highlight the differences in viral resistance dynamics between gene and standard antiretroviral therapies, and identify key factors that impact long-term viral suppression. In particular, we underscore the importance of mutationally-induced viral fitness losses in cells that are not genetically modified, as these can severely constrain the replication of resistant virus. We also propose and investigate a novel treatment strategy that leverages upon gene therapy's unique capacity to deliver different genes to distinct cell populations, and we find that such a strategy can dramatically improve efficacy when used judiciously within a certain parametric regime. Finally, we revisit a previously-suggested idea of improving clinical outcomes by boosting the proliferation of the genetically-modified cells, but we find that such an approach has mixed effects on resistance dynamics. Our results provide insights into the short- and long-term effects of gene therapy and the role of its key properties in the evolution of resistance, which can serve as guidelines for the choice and optimization of effective therapeutic agents.

A primary obstacle to the success of any anti-HIV treatment is HIV's ability to rapidly resist it by generating new viral strains whose vulnerability to the treatment is reduced. Gene therapies represent a novel class of treatments for HIV infection that may supplement or replace present therapies, as they alleviate some of their major shortcomings. The design of gene therapeutic agents that effectively reduce viral resistance can be aided by a quantitative elucidation of the processes by which resistance is acquired following therapy initiation. We developed a computational model that describes a patient's response to therapy and used it to quantify the influence of therapy parameters and strategies on the development of viral resistance. We find that gene therapy induces different clinical conditions and a much slower viral response than present therapies. These dictate different design principles such as a greater significance to the virus' competence in the absence of therapy. We also show that one can effectively delay emergence of resistance by delivering distinct therapeutic genes into separate cell populations. Our results highlight the differences between traditional and gene therapies and provide a basic understanding of how key controllable parameters and strategies affect resistance development.

Stable marking and transgene expression without progression to monoclonality in canine long-term hematopoietic repopulating cells transduced with lentiviral vectors

Lentiviral gene transfer vectors have a number of potential advantages over gammaretroviral vectors including more efficient transduction of nondividing cells, a more favorable integration site profile, and the ability to accommodate large transgenes. Here, we present long-term follow-up data of animals that received lentivirus-transduced CD34-enriched cells. Six long-term surviving dogs were available for analysis. Transgene expression was analyzed from at least 12 months to more than 5 years after transplantation in peripheral blood cells and multiple cell lineages. All animals demonstrated long-term stable transgene expression in peripheral blood myeloid, lymphoid, and red blood cells as well as in platelets. Vector integration sites were analyzed by linear amplification-mediated polymerase chain reaction and showed a polyclonal repopulation pattern in all animals. There was no evidence of any development of monoclonality or leukemia in the animals. The stable long-term multilineage transgene expression, together with detection of the same integration site in myeloid and lymphoid cells, strongly suggests the transduction of long-term repopulating stem cells. Our data demonstrate safe and efficient transduction of multilineage long-term repopulating cells with lentiviral vectors and support the use of such vectors for gene therapy studies in patients.

Large animal models of hematopoietic stem cell gene therapy

Large animal models have been instrumental in advancing hematopoietic stem cell (HSC) gene therapy. Here we review the advantages of large animal models, their contributions to the field of HSC gene therapy and recent progress in this field. Several properties of human HSCs including their purification, their cell-cycle characteristics, their response to cytokines and the proliferative demands placed on them after transplantation are more similar in large animal models than in mice. Progress in the development and use of retroviral vectors and ex vivo transduction protocols over the last decade has led to efficient gene transfer in both dogs and nonhuman primates. Importantly, the approaches developed in these models have translated well to the clinic. Large animals continue to be useful to evaluate the efficacy and safety of gene therapy, and dogs with hematopoietic diseases have now been cured by HSC gene therapy. Nonhuman primates allow evaluation of aspects of transplantation as well as disease-specific approaches such as AIDS (acquired immunodeficiency syndrome) gene therapy that can not be modeled well in the dog. Finally, large animal models have been used to evaluate the genotoxicity of viral vectors by comparing integration sites in hematopoietic repopulating cells and monitoring clonality after transplantation.

Foamy combinatorial anti-HIV vectors with MGMTP140K potently inhibit HIV-1 and SHIV replication and mediate selection in vivo

Highly active antiretroviral therapy has greatly reduced the morbidity and mortality from human immunodeficiency virus (HIV) infection, but AIDS continues to be a serious health problem worldwide. Despite enormous efforts to develop a vaccine, there is still no cure, and alternative approaches including gene therapy should be explored. In this study we developed and compared combinatorial foamy virus (FV) anti-HIV vectors that also express a mutant methylguanine methyltransferase (MGMTP140K) transgene to increase the percentage of gene-modified cells after transplantation. These FV vectors inhibit replication of HIV-1 and also the simian immunodeficiency virus/HIV-1 (SHIV) chimera that can be used in monkey AIDS gene therapy studies. We identified a combinatorial FV vector that expresses 3 anti-HIV transgenes and inhibits viral replication by over 4 logs in a viral challenge assay. This FV anti-HIV vector expresses an HIV fusion inhibitor and two short hairpin RNAs (shRNAs) targeted to HIV-1 tat and rev, and can be produced at high titer (3.8 x 10(7) transducing units ml(-1)) using improved helper plasmids suitable for clinical use. Using a competitive repopulation assay, we show that human CD34(+) cells transduced with this combinatorial FV vector efficiently engraft in a mouse xenotransplantation model, and that the percentage of transduced repopulating cells can be increased after transplantation.

Five decades of progress in haematopoietic cell transplantation based on the preclinical canine model

The preclinical canine model has proved valuable for the development of principles and techniques of haematopoietic cell transplantation (HCT) applicable to human patients. Studies in random-bred dogs concerning the impact of histocompatibility barriers on engraftment and graft-versus-host disease, the kinetics of immunological reconstitution, the efficacy of various pretransplant conditioning regimens, post-transplantation immunosuppression protocols, treatment of malignant diseases, and graft-versus-tumour effects have advanced HCT from an investigational therapy with uncertain clinical benefit half a century ago to an important treatment choice for thousands of patients treated annually in transplantation centres worldwide. More recent preclinical canine studies have resulted in the clinical translation of non-myeloablative, minimally invasive transplantation protocols that have extended allogeneic HCT to include older human patients with malignant and non-malignant, acquired or inherited haematological disorders, and those with comorbid conditions. Here, we review the contributions of the canine model to modern HCT and describe the usefulness of HCT for the treatment of canine haematological disorders.

Protection of stem cell-derived lymphocytes in a primate AIDS gene therapy model after in vivo selection

BACKGROUND

There is currently no effective AIDS vaccine, emphasizing the importance of developing alternative therapies. Recently, a patient was successfully transplanted with allogeneic, naturally resistant CCR5-negative (CCR5Delta32) cells, setting the stage for transplantation of naturally resistant, or genetically modified stem cells as a viable therapy for AIDS. Hematopoietic stem cell (HSC) gene therapy using vectors that express various anti-HIV transgenes has also been attempted in clinical trials, but inefficient gene transfer in these studies has severely limited the potential of this approach. Here we evaluated HSC gene transfer of an anti-HIV vector in the pigtailed macaque (Macaca nemestrina) model, which closely models human transplantation.

METHODS AND FINDINGS

We used lentiviral vectors that inhibited both HIV-1 and simian immunodeficiency virus (SIV)/HIV-1 (SHIV) chimera virus infection, and also expressed a P140K mutant methylguanine methyltransferase (MGMT) transgene to select gene-modified cells by adding chemotherapy drugs. Following transplantation and MGMT-mediated selection we demonstrated transgene expression in over 7% of stem-cell derived lymphocytes. The high marking levels allowed us to demonstrate protection from SHIV in lymphocytes derived from gene-modified macaque long-term repopulating cells that expressed an HIV-1 fusion inhibitor. We observed a statistically significant 4-fold increase of gene-modified cells after challenge of lymphocytes from one macaque that received stem cells transduced with an anti-HIV vector (p<0.02, Student's t-test), but not in lymphocytes from a macaque that received a control vector. We also established a competitive repopulation assay in a second macaque for preclinical testing of promising anti-HIV vectors. The vectors we used were HIV-based and thus efficiently transduce human cells, and the transgenes we used target HIV-1 genes that are also in SHIV, so our findings can be rapidly translated to the clinic.

CONCLUSIONS

Here we demonstrate the ability to select protected HSC-derived lymphocytes in vivo in a clinically relevant nonhuman primate model of HIV/SHIV infection. This approach can now be evaluated in human clinical trials in AIDS lymphoma patients. In this patient setting, chemotherapy would not only kill malignant cells, but would also increase the number of MGMTP140K-expressing HIV-resistant cells. This approach should allow for high levels of HIV-protected cells in AIDS patients to evaluate AIDS gene therapy.

Long-term transduction of miniature pig parotid glands using serotype 2 adeno-associated viral vectors

BACKGROUND

Previously, using an adenoviral vector, we showed that miniature pigs could provide a valuable and affordable large animal model for pre-clinical gene therapy studies to correct parotid gland radiation damage. However, adenoviral vectors lead to short-term transgene expression and, ideally, a more stable correction is required. In the present study, we examined the suitability of using a serotype 2 adeno-associated viral (AAV2) vector to mediate more stable gene transfer in the parotid glands of these animals.

METHODS

Heparan sulfate proteoglycan was detected by immunohistochemistry. beta-galactosidase expression was determined histochemically. An AAV2 vector encoding human erythropoietin (hEpo) was administered via Stensen's duct. Salivary and serum hEpo levels were measured using an enzyme-linked immunosorbent assay. Serum chemistry and hematological analyses were performed and serum antibodies to hEpo were measured throughout the study. Vector distribution was determined by a quantitative polymerase chain reaction.

RESULTS

Transgene expression was vector dose-dependent, with high levels of hEpo being detected for up to 32 weeks (i.e. the longest time studied). hEpo reached maximal levels during weeks 4-8, but declined to approximately 25% of these values by week 32. Haematocrits were elevated from week 2. Transduced animals exhibited low serum anti-hEpo antibodies (1 : 8-1 : 16). Vector biodistribution at animal sacrifice revealed that most copies were in the targeted parotid gland, with few being detected elsewhere. No consistent adverse changes in serum chemistry or hematology parameters were seen.

CONCLUSIONS

AAV2 vectors mediate extended gene transfer to miniature pig parotid glands and should be useful for testing pre-clinical gene therapy strategies aiming to correct salivary gland radiation damage.

Long-term polyclonal and multilineage engraftment of methylguanine methyltransferase P140K gene-modified dog hematopoietic cells in primary and secondary recipients

Overexpression of methylguanine methyltransferase P140K (MGMTP140K) has been successfully used for in vivo selection and chemoprotection in mouse and large animal studies, and has promise for autologous and allogeneic gene therapy. We examined the long-term safety of MGMTP140K selection in a clinically relevant dog model. Based on the association of provirus integration and proto-oncogene activation leading to leukemia in the X-linked immunodeficiency trial, we focused our analysis on the distribution of retrovirus integration sites (RIS) relative to proto-oncogene transcription start sites (TSS). We analyzed RIS near proto-oncogene TSS before (n = 157) and after (n = 129) chemotherapy in dogs that received MGMTP140K gene-modified cells and identified no overall increase of RIS near proto-oncogene TSS after chemotherapy. We also wanted to determine whether in vivo selected cells retained fundamental characteristics of hematopoietic stem cells. To that end, we performed secondary transplantation of MGMTP140K gene-modified cells after in vivo selection in dog leukocyte antigen (DLA)-matched dogs. Gene-modified cells achieved multilineage repopulation, and we identified the same gene-modified clone in both dogs more than 800 and 900 days after transplantation. These data suggest that MGMTP140K selection is well tolerated and should allow clinically for selection of gene-corrected cells in genetic or infectious diseases or chemoprotection for treatment of malignancy.

Evaluation of the effect of autologous mesenchymal stem cell injection in a large-animal model of bilateral kidney ischaemia reperfusion injury

OBJECTIVES

Adult mesenchymal stem cells (MSC) have been proven to be of benefit to the kidney in different experimental models of renal injuries. All studies have been performed in valuable rodent models, but the relevance of these results to large mammals and ultimately, to humans remains unknown. Therefore, the aim of this study was to investigate the effect of MSC transplantation in an alternative ovine large-animal model of bilateral kidney ischaemia reperfusion injury.

MATERIAL AND METHODS

Sheep were divided into three groups: one sham-operated group and two groups submitted to renal bilateral ischaemia for 60 min. Animals with ischaemia reperfusion injury were treated with injection of autologous MSCs or with vehicle medium.

RESULTS

The model sheep presented with renal histological manefestations that closely resembled lesions seen in patients. Transplanted MSCs were found in glomeruli but not in tubules and did not express glomerular cell markers (podocin, von Willebrand factor), but functional evaluation showed no beneficial effect of MSC infusion. Morphological and molecular analyses corroborated the functional results. MSCs did not repair kidney parenchyma and failed to modulate cell death and proliferation or cytokine release (tumour necrosis factor-alpha, vascular endothelial growth factor alpha (VEGF-alpha), Bcl-2, caspase).

CONCLUSION

In this unique autologous large-animal model, MSCs did not exhibit reparative or paracrine protective properties.

Development and use of DNA archives at veterinary teaching hospitals to investigate the genetic basis of disease in dogs

The DNA archives developed at veterinary medical teaching hospitals will be important resources for mapping disease loci and identifying underlying genes. The most important feature of a DNA archive is accurate identification or exclusion of diseases in each animal. Such archives will be complimentary resources to tissue banks that are currently available.

Overnight transduction with foamyviral vectors restores the long-term repopulating activity of Fancc-/- stem cells

Fanconi anemia (FA) is a complex genetic disorder characterized by congenital abnormalities, bone marrow failure, and myeloid malignancies. Identification of 13 FA genes has been instrumental to explore gene transfer technologies aimed at correction of autologous FA-deficient stem cells. To date, 3 human FA stem cell gene therapy trials with standard 4-day transduction protocols using gammaretroviral vectors failed to provide clinical benefit. In addition, 2- to 4 day ex vivo manipulation of bone marrow from mice containing a disruption of the homologue of human FANCC (Fancc) results in a time-dependent increase in apoptosis and a risk for malignant transformation of hematopoietic cells. Here, we show that a 14-hour transduction period allows a foamyviral vector construct expressing the human FANCC cDNA to efficiently transduce murine FA stem cells with 1 to 2 proviral integrations per genome. Functionally, the repopulating activity of Fancc(-/-) stem cells from reconstituted mice expressing the recombinant FANCC transgene was comparable with wild-type controls. Collectively, these data provide evidence that short-term transduction of c-kit(+) cells with a foamyviral vector is sufficient for functional correction of a stem cell phenotype in a murine FA model. These data could have implications for future gene therapy trials for FA patients.

Development of gene therapy for blood disorders

The concept of introducing genes into human cells for therapeutic purposes developed nearly 50 years ago as diseases due to defects in specific genes were recognized. Development of recombinant DNA techniques in the 1970s and their application to the study of mouse tumor viruses facilitated the assembly of the first gene transfer vectors. Vectors of several different types have now been developed for specific applications and over the past decade, efficacy has been demonstrated in many animal models. Clinical trials began in 1989 and by 2002 there was unequivocal evidence that children with severe combined immunodeficiency could be cured by gene transfer into primitive hematopoietic cells. Emerging from these successful trials was the realization that proto-oncogene activation by retroviral integration could contribute to leukemia. Much current effort is focused on development of safer vectors. Successful gene therapy applications have also been developed for control of graft-versus-host disease and treatment of various viral infections, leukemias, and lymphomas. The hemophilias seem amenable to gene therapy intervention and informative clinical trials have been conducted. The hemoglobin disorders, an early target for gene therapy, have proved particularly challenging although ongoing research is yielding new information that may ultimately lead to successful clinical trials.

Bone marrow-derived cells exhibiting lung epithelial cell characteristics are enriched in vivo using methylguanine DNA methyltransferase-mediated drug resistance

Previous studies have suggested that donor bone marrow-derived cells can differentiate into lung epithelial cells at low frequency. We investigated whether we could enrich the number of donor-derived hematopoietic cells that have type II pneumocyte characteristics by overexpression of the drug resistance gene methylguanine DNA methyltransferase (MGMT). MGMT encodes O(6)-alkylguanine DNA alkyltransferase (AGT), a drug resistance protein for DNA damage induced by N,N'-bis(2-chloroethyl)-N-nitrosourea (BCNU), and the mutant P140K MGMT confers resistance to BCNU and the AGT inactivator O(6)-benzylguanine (BG). For this study, we used two MGMT selection models: one in which donor cells had a strong selection advantage because the recipient lung lacked MGMT expression, and another in which drug resistance was conferred by gene transfer of P140K MGMT. In both models, we saw an increase in the total number of donor-derived cells in the lung after BCNU treatment. Analysis of single-cell suspensions from 28 mice showed donor-derived cells with characteristics of type II pneumocytes, determined by surfactant protein C (SP-C) expression. Furthermore, an increase in the percentage of donor-derived SP-C cells was noted after BCNU or BG and BCNU treatment. This study demonstrates that bone marrow cells expressing MGMT can engraft in the lung and convert into cells expressing the type II pneumocyte protein SP-C. Furthermore, these cells can be enriched in response to alkylating agent-mediated lung injury. These results suggest that expression of MGMT could enhance the capacity of bone marrow-derived cells to repopulate lung epithelium, and when used in combination with a gene of interest, MGMT could have therapeutic applications.

Effective gene therapy of mice with congenital erythropoietic porphyria is facilitated by a survival advantage of corrected erythroid cells

Achieving long-term expression of a therapeutic gene in a given hematopoietic lineage remains an important goal of gene therapy. Congenital erythropoietic porphyria (CEP) is a severe autosomal-recessive disorder characterized by a deficiency in uroporphyrinogen III synthase (UROS), the fourth enzyme of the heme biosynthetic pathway. We used a recently obtained murine model to check the feasibility of gene therapy in this disease. Lentivirus-mediated transfer of the human UROS cDNA into hematopoietic stem cells (HSCs) from Uros(mut248) mice resulted in a complete and long-term enzymatic, metabolic, and phenotypic correction of the disease, favored by a survival advantage of corrected red blood cells. These results demonstrate that the cure of this mouse model of CEP at a moderate transduction level supports the proof of concept of a gene therapy in this disease by transplantation of genetically modified hematopoietic stem cells.

Transduction of rhesus macaque hematopoietic stem and progenitor cells with avian sarcoma and leukosis virus vectors

Genome-wide integration site analyses showed that Moloney murine leukemia virus (MoMLV)- and lentivirus-derived vectors integrate preferentially into the coding regions of genes, posing a risk of insertional mutagenesis. Avian sarcoma and leukosis viruses (ASLVs) were previously reported to have a weak preference for gene-coding regions in a cell line study as compared with human immunodeficiency virus and MoMLV; however, thus far these vectors have not been studied for their potential efficacy in transduction of hematopoietic progenitor and stem cells. In this study we investigated for the first time the ability of ASLV-derived RCAS (replication-competent ALV LTR [avian leukosis virus long terminal repeat] with a splice acceptor) vectors to transduce rhesus macaque hematopoietic progenitors and long-term repopulating cells, in an autologous transplantation model. RCAS vectors can efficiently and stably transduce rhesus macaque CD34+ hematopoietic progenitor cells with an efficiency of transduction of up to 34% ex vivo. In two animals transplanted with RCAS vector-transduced autologous CD34+ cells, highly polyclonal hematopoietic reconstitution with sustained gene-marking levels in myeloid and lymphoid lineages was observed up to 18 months post-transplantation. These findings are encouraging and suggest that this vector system should be explored and further optimized for gene therapy applications targeting hematopoietic stem and progenitor cells.

O6-methylguanine-DNA-methyltransferase (MGMT) gene therapy targeting haematopoietic stem cells: studies addressing safety issues

As haematopoietic stem cell gene therapy utilizing O(6)-methylguanine-DNA-methyltransferase has reached the clinical stage, safety-related questions become increasingly important. These issues concern insertional mutagenesis of viral vectors, the acute toxicity of pre-transplant conditioning protocols and in vivo selection regimens as well as potential genotoxic side effects of the alkylating drugs administered in this context. To address these questions, we have investigated toxicity-reduced conditioning regimens combining low-dose alkylator application with sublethal irradiation and have analysed their influence on engraftment and subsequent selectability of transduced haematopoietic stem cells. In addition, a strategy to monitor the acute and long-term genotoxic effects of drugs with high guanine-O(6) alkylating potential, such as chloroethylnitrosoureas or temozolomide is introduced. For this purpose, assays were implemented which allow an assessment of the generation and fate of primary drug-induced adducts as well as their long-term effect on chromosomal integrity at the single cell level.

Vector design for expression of O6-methylguanine-DNA methyltransferase in hematopoietic cells

Enhancing DNA repair activity of hematopoietic cells by stably integrating gene vectors that express O(6)-methylguanine-DNA-methyltransferase (MGMT) is of major interest for innovative approaches in tumor chemotherapy and for the control of hematopoietic chimerism in the treatment of multiple other acquired or inherited disorders. Crucial determinants of this selection principle are the stringency of treatment with O(6)-alkylating agents and the level of transgenic MGMT expression. Attempts to generate clinically useful MGMT vectors focus on the design of potent expression cassettes, an important component of which is formed by enhancer sequences that are active in primitive as well as more differentiated hematopoietic cells. However, recent studies have revealed that vectors harboring strong enhancer sequences are more likely to induce adverse events related to insertional mutagenesis. Safety-improved vectors that maintain high levels of MGMT expression may be constructed based on the following principles: choice of enhancer-promoter sequences with relatively mild long-distance effects despite a high transcription rate, improved RNA processing (export, stability and translation), and protein design. The need for optimizing MGMT protein design is supported by recent observations suggesting that the P140K mutant of MGMT, developed to be resistant to inhibitors such as O(6)-benzylguanine, may confer a selective disadvantage when expressed at high levels. Here, we provide a review of the literature exploring MGMT expression vectors for bone marrow chemoprotection, and describe experimental evidence suggesting that high expression of MGMT P140K induces a selective disadvantage in the absence of alkylating agents. We conclude that the appropriate design of expression vectors and MGMT protein features will be crucial for the long-term prospects of this promising selection principle.

Development of a synthetic promoter for macrophage gene therapy

Macrophages have the potential to deliver therapeutic genes to many target tissues. Macrophage-specific synthetic promoters (SPs) generated by random ligation of myeloid/macrophage cis elements had activity up to 100-fold that of a native macrophage promoter in macrophage cell lines, but were minimally active in nonmyeloid cells. Mouse bone marrow cells (BMCs) transduced ex vivo with lentivectors expressing green fluorescent protein (GFP) driven either by an SP (SP-GFP) or a cytomegalovirus (CMV) promoter (CMV-GFP) were used for syngeneic transplantation of lethally irradiated mice. Blood leukocytes showed stable GFP expression for up to 15 months after transplantation. SP-GFP expression was selective for CD11b+ macrophages, whereas CMV-GFP expression was observed in erythrocytes, as well as in both CD11b+ and CD11b- leukocytes. Furthermore, SP-GFP expression was much stronger than CMV-GFP expression in CD11b+ macrophages. apoE-/- BMCs transduced with the lentiviral vector encoding human apoE were used to transplant apoE-/- mice. Macrophage expression of apoE from 10 to 26 weeks of age significantly reduced atherosclerotic lesions in recipient apoE-/- mice. Thus, the novel SPs, especially when combined with lentivectors, are useful for macrophage-specific delivery of therapeutic genes.